Structurally, Castle Valley is part of a large collapsed salt anticline that includes Paradox Valley to the southeast on the other side of the La Sal Mountains. Over 300 million years ago, thick layers of salt were deposited in the area under marine conditions. These layers were subsequently buried under sediments. The entire area was lifted in the late Tertiary to create the Colorado Plateau. Over time, erosion of the sedimentary layers allowed ground and surface water to contact and dissolve the salt layers. Overlying rock strata collapsed and eroded, forming Castle Valley. High angle fault systems exist on both sides of Castle Valley as a result of the collapse process. Much later, a mere 24 million years ago, the La Sal Mountains and Castle Valley's Round Mountain rose up as a result of underground volcanic activities. Officially called laccoliths, the La Sals are a kind of volcanic blister created as rising magma was not released through the surface but forced its way into overlying sedimentary layers, lifting them up by over 6,000 feet. The Valley floor has been filled over eons by sedimentary processes, mass movement and stream deposits.
